The present invention relates to a method for detecting and processing magnetic resonance (MR) signals, wherein different phase encoding data items are provided to a plurality of MR echo signals generating by magnetic resonance within a subject, thereby to shorten the time required for scanning a subject.
Conventionally, to form a tomographic image of a subject, MR signals generating by the magnetic resonace caused within the subject are processed by means two-dimensional Fourier transform. Each MR echo signal can be given: EQU f(.xi.,.eta.)=.intg..intg.I(x,y)exp(j(.xi.x+.eta.y))dxdy (1)
where EQU .xi.=.gamma..multidot.Gx.multidot.tx EQU .eta.=.gamma..multidot.Gy.multidot.ty
In the above equations, .gamma. is the gyromagnetic ratio, Gx is the intensity of the gradient magnetic field in the x direction, Gy is the intensity of the gradient magnetic field in the y direction, tx is the time period during which the magnetic field is maintained at Gx, ty is the time period during which the magnetic field is maintained at Gx, and I(x,y) is the spin density distribution in a slice within the subject. Hence, MR echo signal f(.xi.,.eta.) is expressed as the product of (Gx, Gy) and (tx, ty). To form a tomogram of the slice, MR echo signals reflected to (.xi.,.eta.), generally called "Fourier plane" are acquired and subjected to the two-dimensional Fourier transform which is given: EQU I(x,y)=.intg..intg.f(.xi.,.eta.)exp(-j(.xi.x+.eta.y)d.xi.d.eta.(2)
As is shown in FIGS. 1A to 1D, a 90.degree.-pulse and gradient magnetic field Gz are used to select a desired slice portion of the subject and excite the spinning nuclei existing in this portion of the subject, whereas a 180.degree.-pulse and gradient magnetic field Gx are used to acquire an MR echo signal representing an excited spinning nucleus. The frequency components of the MR echo signal determined by the distribution of the nuclear magnegic moment in the x direction. Gradient magnetic field Gz and gradient magnetic field Gx, which intersects at right angles with gradient magentic field Gy, are used to gradually change the amplitude of field Gy from a negative value to a positive value, thereby to provide phase-encoding information.
Every time the spinning nuclei excite by application of a 90.degree.-pulse, the amplitude of gradient magnetic field Gy is changed from -N to N, as is shown in FIGS. 2A and 2B, thereby acquiring first and second echo signals. The spinning nuclei are excited 2N +1 times, whereby MR echo siganls are acquired. These MR echo signals are subjected to the two-dimensional Fourier transform, thereby to form two MR images.
The conventional image-forming method described above requires a relatively long scanning time since two MR echo signal data concerning two Fourier planes are acquired by exciting the spinning nuclei, 2N +1 times, to form a tomographic image of the subject. Consequently, it takes a long period of time to form a tomographic image of one slice. Therefore, it is desired that a method be taken to shorten the scanning time.